Assistance systems may be used with steering device of vehicles, such as motor vehicles. Assistance systems may range from, for example, power steering systems to independently acting steering systems for use in parking situations. Such systems may be active in the steering process during travel to increase driving comfort and safety for vehicle occupants.
Power steering systems may differ in the form of their respective drives. Customary hydraulically operated power steering systems (HPS=“Hydraulic Power Steering”) serve primarily for providing pure power assistance steering. They have a hydraulic pump coupled to the vehicle engine in such a way that it transmits torque. The hydraulic pump feeds a continuous volume flow through a hydraulic cylinder including a piston which has pressure applied to both of its sides. In a directionally stable state, this piston is kept virtually pressureless. In the case of a steering lock, the pressure can be increased on one side, as required as a function of the speed, to facilitate movement of the steering device in a necessary direction.
In contrast, other systems have a power steering system with an electrically driven motor (EPS=“Electric Power Steering”). By eliminating the hydraulic pump, these systems offer corresponding energetic advantages. In addition, the electric pump of the systems is activated only when necessary so there is no continuous power loss for the vehicle engine when it is operating.
EPS systems permit the possibility of combination with further assistance systems. Thus, it is possible to signal to the driver, for example, driving states that have been detected by the assistance system. Furthermore, such a system can perform active interventions in a steering process to assist and/or relieve the driver. Such systems may also be distinguished in terms of their type of drive. In addition to electrohydraulic operation (EHPS=“Electro-Hydraulic Power Steering”), a purely electromechanical implementation may be used (EPAS=“Electro Power Assisted Steering”). EHPS systems retain the hydraulically assisted steering system. However, in contrast to simple EPS systems, EHPS systems have an electrically operated hydraulic pump which acts independently of the vehicle engine. In contrast, EPAS systems dispense entirely with a hydraulic system by virtue of the fact that they have an electric motor, which is coupled directly to the steering system.
Conventional power assist systems and methods described above relieve and assist the driver with respect to the force to be applied, as well as with respect to monitoring a vehicle that is equipped in this way. In particular, varying external influences impose stringent requirements upon such systems to provide a completely adaptive complementary interplay between actions and reactions by driver and the auxiliary systems. For example, conventional systems correct for driver input and can also correct for vehicle conditions and/or road conditions. However, in existing power steering assist systems and methods, such corrections are not made sufficiently quickly to respond, for example, to continuously changing conditions, such as road inclination/camber/crown. Thus, in certain power steering assist systems that employ pull drift compensation (PDC), such corrections compensate only for permanent pull (e.g., caused by “permanent” conditions, such as vehicle misalignment) and not for short-term pull (created, for example, by changing road inclination).
For these reasons, it may be desirable to create a power steering assist system with PDC capability to correct for both long-term and short-term pull/drift.